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MAZER
Page 289
Energy is needed to break bonds in reactant molecules.
Energy is released when bonds are formed in product molecules.
By adding up the bond energies in the reactants and products, you
can determine whether energy will be released or absorbed.
Sometimes more energy is released when products form than is
needed to break the bonds in the reactants. Then energy is released
during the reaction. A reaction in which energy is released is called
an exothermic reaction.
Sometimes more energy is required to break the bonds in the reactants than is released when the products form. Then energy must be
added to the reaction. That is, the reaction absorbs energy. A reaction
in which energy is absorbed is called an endothermic reaction.
These types of energy changes can also be observed in different
physical changes such as dissolving or changing state. The state
change from a liquid to a solid, or freezing, releases energy—this is
an exothermic process. The state change from a solid to a liquid, or
melting, absorbs energy—this is an endothermic process.
Check Your Reading
The white clouds of
water vapor are formed
by the exothermic reaction between hydrogen
and oxygen.
How are exothermic and endothermic reactions different?
Exothermic reactions release energy.
2H2 + O2
Exothermic chemical reactions often produce an increase in temperature. In exothermic reactions, the bond energies of the reactants are
less than the bond energies of the products. As a result, less energy is
needed to break the bonds in the reactants than is released during the
formation of the products. This energy difference between reactants
and products is often released as heat. The release of heat causes a
change in the temperature of the reaction mixture.
Even though energy is released by exothermic reactions, some
energy must first be added to break bonds in the reactants. In exothermic reactions, the formation of bonds in the products releases more
energy. Overall, more energy is released than is added.
2H2O
The thermite reaction
releases enough heat
to weld pieces of iron
together.
Some reactions are highly exothermic. These reactions produce a
great deal of heat and significantly raise the temperature of their surroundings. One example is the reaction of powdered aluminum metal
with a type of iron oxide. This reaction is known as the thermite reaction. The equation for this reaction is
2Al + Fe2O3
Al2O3 + 2Fe
This reaction releases enough heat to melt the iron that is produced.
In fact, this reaction is used to weld iron rails together.
Check Your Reading
What is evidence for an exothermic chemical reaction?
Chapter 9: Chemical Reactions 289
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